Generally, a method of preparing a concentrate of fine ceria particles with relatively high purity, includes providing fine and dry ceria particles with the relatively high purity, and thickly dispersing the particles in a medium, such as water, or synthesizing the fine ceria particles in the medium, such as water, in a relatively high concentration.
Meanwhile, a process of preparing the fine ceria particles is classified into gas, liquid, and solid phase processes according to a phase in which the particles are produced.
In the case of the gas phase process, a cerium metal or a cerium metal precursor is gasified, and then reacts with oxygen and the like. The gas phase process may be classified into various processes, such as a flame combustion pyrolysis process, a laser gasification process, a plasma gasification process, and a spray pyrolysis process, according to a gasification manner or a reaction manner. However, the gas phase process is disadvantageous in that a great amount of energy is consumed, the costs of equipments are relatively high, and the productivity is relatively low, even though it has some advantages in that a relatively simple process is ensured and fine particles with a uniform particle size are produced. Hence, the fine ceria particles produced according to the gas phase process are costly.
A calcination process, regarded as a representative process of the solid phase process, is a traditional process of preparing metal oxide particles, in which cerium carbonate used as a precursor is pyrolyzed for a relatively long time in a high temperature furnace, oxidized to produce ceria, and crystallized for a relatively long time to produce powder. In this regard, the powder is crushed into fine particles.
WO 99/31195 and Japanese Pat. Laid-Open Publication Nos. Hei. 10-106993, 10-154672, and 12-160136 disclose a technology of preparing ceria particles for semi-conductor planarization, in which cerium carbonate as a raw material is heated at high temperatures for a long time in a calcination furnace, subjected to a dry crushing process to produce particles with a particle size of μm unit. At this time, the particles are subjected to a solution high-speed jet milling process to become fine, and the fine particles are filtered to accomplish a CMP slurry. However, the technology is disadvantageous in that the raw material is heated at high temperatures for a long time, impurities are easily mixed with the particles during the crushing and high-speed milling processes, a great amount of energy is consumed, and a production time of the ceria particles is very long. Other disadvantages are that coarse particles with the particle size of 1 μm or more exist in the final ceria particles because it is difficult to desirably control the particle size to convert all primary particles into the fine particles. Accordingly, in the above technology, it is necessary to filter the end CMP slurry, bringing about the loss of the particles.
As for a mechano-chemical process (MCP), a surface of a precursor, such as cerium chloride, is activated (high temperature and the like) by a high-speed and high-energy mechanical stimulation (high-speed ball milling), thereby completing a desired reaction. However, the MCP has disadvantages in that impurities are added into desired particles due to balls and vials during the ball milling process, it is very difficult to wash a large amount of alkali metal salt added as an additive to the particles, a reaction time is very long, and production costs of the particles are very high because it is necessary to conduct a calcination process.
Meanwhile, with respect to a method of preparing fine ceria particles for semi-conductor planarization use according to the liquid phase process, WO 97/29510 suggests three methods of preparing the ceria particles: a first method of preparing ceria, including milling a nonaqueous cerium salt compound (cerium carbonate) with a valence of 3, dispersed in water, oxidizing the milled cerium salt compound using an oxidizing agent (hydrogen peroxide), and hydrothermally treating the resulting compound, a second method of preparing ceria, including oxidizing the aqueous cerium salt compound (cerium nitrate) with the valence of 3 using the oxidizing agent, and hydrothermally treating the resulting compound, and a third method of preparing ceria, including controlling the pH of the aqueous cerium salt compound (ammonium cerium nitrate) with the valence of 4 so as to enable the cerium salt compound to be alkaline to produce cerium hydroxide, and hydrothermally treating the cerium hydroxide.
However, the above three methods are disadvantageous in that a produced amount of ceria is limited because ceria is produced according to a batch process. Additionally, when the resulting ceria particles contain waste acidic and alkaline materials, purification, washing, and drying processes of the ceria particles are inevitably required, and thus, the methods are complicated and it is difficult to massproduce the ceria particles. Further, the methods have the disadvantages of a large particle size and a wide particle size distribution.
Meanwhile, a supercritical hydrothermal synthesis, which is a kind of a hydrothermal process, has been studied by Ajri et al. in Japanese, as recited in Ind. Eng. Chem. Res. Vol. 39, 4901-4907 (2000), in which a water-soluble cerium salt is treated under a condition of supercritical water (Tc≧374° C., Pc≧22.4 MPa) to easily produce fine nano-sized particles.
WO 87/04421 and U.S. Pat. No. 5,635,154 disclose a process of preparing fine metal oxide particles according to batch and continuous type of supercritical hydrothermal syntheses. However, the batch type of supercritical hydrothermal synthesis has disadvantages in that a reaction is conducted for a relatively long time (tens or more min), and thus, it is difficult to desirably control a particle size and a particle size distribution is wide. As for the continuous type of supercritical hydrothermal synthesis, it has advantages in that the reaction is completed within a relatively short time of 1 to 60 sec, it is not necessary to conduct a calcination process, purity of a product is high, and it is easy to control the crystallization, a particle size, and a shape of the fine particles. However, the batch and continuous type of supercritical hydrothermal syntheses are problematic in that it is necessary to separate the particles from a solution phase and to wash the particles because waste acid, such as nitric acid, is generated during the reaction.
U.S. Pat. Nos. 5,433,878, 5,480,630, and 5,635,154 disclose a technology of preparing fine metal oxide particles, in which a water-soluble metal salt compound is reacted in a continuous tubular reactor at 200° C. or higher under pressure of 250 to 500 kg/cm2 for 1 to 10 min. However, these U.S. patents all have the disadvantage of the generation of waste acid as a by-product.
An effort has been made to avoid the above disadvantages. For example, reference may be made to the patent which has been made by the applicant of the present invention, Korean Pat. Application No. 2002-0041376 (a method of preparing fine metal oxide particles), which discloses a continuous supercritical, hydrothermal synthesis. In this regard, a metal salt aqueous solution and an aqueous solution of ammonia are reacted with each other in a reactor to produce fine metal oxide particles, with nitric acid as a by-product. At this time, nitric acid is decomposed while the fine metal oxide particles are produced, thereby significantly reducing a content of nitrogen-containing compounds in waste water generated during a reaction of the metal salt aqueous solution and an aqueous solution of ammonia. With respect to this, a concentration process, such as centrifuge or filtering processes, an oven drying process, a lyophilizing process, or a spray drying process may be conducted to recover the particles from the metal oxide solution. However, the above drying processes are problematic in that considerable time and costs are consumed to recover the dry particles. Particularly, the above Korean patent is disadvantageous in that it is difficult to massproduce the fine metal oxide particles according to the centrifuge and lyophilizing processes, physical properties of the particles may be changed to agglomerate or aggregate the fine metal oxide particles during the oven drying process, and it is necessary to crush a dried cake to produce metal oxide powder.